SPECTROSCOPIC STUDY OF GRO J1655-40 - THE OUTBURST AND THE DECLINE

We have analysed 53 optical spectra (40 taken during the outburst peak and 13 during the decline) of the Soft X-ray Transient GRO J1655-40 ( =X-Ray Nova Scorpii 1994). The H-alpha flux during outburst shows a pe riodicity of 1.3 days, which is almost exactly one half of the one obt ained by Bailyn et al. (1995b) from radial velocity variations. A poss ible explanation for this behaviour is provided. We also notice that t he spectral emission features shrink or turn into absorption lines wit h an emission core during the decline. This is typical of Soft X-ray T ransients and suggests that an expansion of the disk takes place after the outburst peak phase.